international computer institute, izmir, turkey relational model
DESCRIPTION
International Computer Institute, Izmir, Turkey Relational Model. Asst.Prof.Dr. İlker Kocabaş UBİ502 at http://ube.ege.edu.tr/~ikocabas/teaching/ubi502/index.html. The Relational Model. Structure of Relational Databases Structure of a relation Structure of a database Keys - PowerPoint PPT PresentationTRANSCRIPT
International Computer Institute, Izmir, TurkeyInternational Computer Institute, Izmir, Turkey
Relational ModelRelational Model
Asst.Prof.Dr.İlker Kocabaş
UBİ502 at http://ube.ege.edu.tr/~ikocabas/teaching/ubi50
2/index.html
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The Relational ModelThe Relational Model
Structure of Relational Databases Structure of a relation
Structure of a database
Keys
Relational Algebra Relational operators
Example queries
Natural join
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Cartesian ProductCartesian Product
let D1, D2, …. Dn be entity sets (or domains)
The Cartesian product of D1, D2, …. Dn, written D1 x D2 x … x Dn
is the set of n-tuples over combinations of entities Example: if
customer-name = {Jones, Smith}customer-street = {Main, North}customer-city = {Harrison}
Then customer-name x customer-street x customer-city =
{ (Jones, Main, Harrison),(Smith, Main, Harrison),(Jones, North, Harrison),(Smith, North, Harrison) }
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Structure of a RelationStructure of a Relation
a relation r is a subset of D1 x D2 x … x Dn
i.e. a set of n-tuples (a1, a2, …, an) where each ai Di
Example: if
customer-name = {Jones, Smith, Curry, Lindsay}customer-street = {Main, North, Park}customer-city = {Harrison, Rye, Pittsfield}
Then r = { (Jones, Main, Harrison), (Smith, North, Rye), (Curry, North, Rye), (Lindsay, Park, Pittsfield) }
is a relation over customer-name x customer-street x customer-city A relation is a set, therefore unordered (more about this later…)
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Attribute TypesAttribute Types
Each attribute of a relation has a name
Names are unique within any given relation
The set of allowed values for each attribute is called the domain of the attribute
Attribute values are (normally) required to be atomic, that is, indivisible multivalued attribute values are not atomic (e.g. phone-number)
composite attribute values are not atomic (e.g. address)
The special value null is a member of every domain
The null value causes complications in the definition of many operations null value is different from empty and space values.
we ignore the effect of null values for now
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Relation SchemaRelation Schema
A1, A2, …, An are attributes
R = (A1, A2, …, An ) is a relation schema
E.g. Customer-schema = (customer-name, customer-street, customer-city)
r(R) is a relation on the relation schema R
E.g. customer (Customer-schema)
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Relation InstanceRelation Instance
The current values (relation instance) of a relation are specified by a table
An element t of r is called a tuple
Each tuple is represented by a row in a table
JonesSmithCurry
Lindsay
customer-name
MainNorthNorthPark
customer-street
HarrisonRyeRye
Pittsfield
customer-city
customer
attributes(or columns)
tuples(or rows)
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Relations are UnorderedRelations are Unordered Order of tuples is irrelevant (tuples may be stored in an arbitrary order)
The following two tables represent the same relation:
No significance to ordering
because both tables represent sets
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Structure of a DatabaseStructure of a Database
A database consists of one or more relations
Information about an enterprise is broken down into its components
Each relation stores just part of the information
E.g.: account : information about accounts depositor : information about which customer owns which account customer : information about customers
Storing all information as a single relation such as bank(account-number, balance, customer-name, ..)results in repetition of information (e.g. two customers own an account)
the need for null values (e.g. represent a customer without an account)
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The The customer customer RelationRelation
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The The depositor depositor RelationRelation
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E-R Diagram for the Banking EnterpriseE-R Diagram for the Banking Enterprise
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E-R Diagram for the Banking EnterpriseE-R Diagram for the Banking Enterprise
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SuperkeysSuperkeys
Let R be the relation schema, and let K R
K is a superkey of R if values for K are sufficient to identify a unique tuple of each possible relation r(R)
possible = a relation r that could exist in the enterprise we are modelling.
E.g. {customer-name,customer-street}
E.g. {customer-name}
Both are superkeys of Customer, if no two customers can possibly have the same name.
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Candidate KeysCandidate Keys
K is a candidate key if: K is a superkey
K is minimal (contains no superkeys)
{customer-name} is a candidate key for Customer it is a superkey
(assuming no two customers can have the same name)
no subset of K is a superkey.
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Determining Keys from E-R SetsDetermining Keys from E-R Sets
Strong entity set. The primary key of the entity set becomes the primary key of the relation.
Weak entity set. The primary key of the relation consists of the union of the primary key of the strong entity set and the discriminator of the weak entity set.
Relationship set. The union of the primary keys of the related entity sets becomes a super key of the relation. For binary many-to-one relationship sets,
the primary key of the “many” entity set becomes the relation’s primary key.
For one-to-one relationship sets,the relation’s primary key can be that of either entity set.
For many-to-many relationship sets, the union of the primary keys becomes the relation’s primary key
√
√ √
? ?
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Schema Diagram for the Banking EnterpriseSchema Diagram for the Banking Enterprise
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Relational AlgebraRelational Algebra
Six basic operators select
project
union
set difference
Cartesian product
rename
The operators take two or more relations as inputs and give a new relation as a result.
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Select OperationSelect Operation
Notation: p(r) p is called the selection predicate
Defined as:
p(r) = {t | t r and p(t)}
Where p is a formula in propositional calculus consisting of terms connected by : (and), (or), (not)Each term is one of:
<attribute> op <attribute> or <constant>
where op is one of: =, , >, . <. Example of selection:
branch-name=“Perryridge”(account)
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Select Operation – ExampleSelect Operation – Example
• Relation r A B C D
1
5
12
23
7
7
3
10
A=B D > 5 (r) A B C D
1
23
7
10
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Project Operation – ExampleProject Operation – Example
Relation r :A B C
10
20
30
40
1
1
1
2
A C
1
1
1
2
=
A C
1
1
2
A,C (r)
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Project OperationProject Operation
Notation:
A1, A2, …, Ak (r)
where A1, A2 are attribute names and r is a relation name.
The result is defined as the relation of k columns obtained by erasing the columns that are not listed
Duplicate rows removed from result, since relations are sets
E.g. To eliminate the branch-name attribute of account
account-number, balance (account)
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Union Operation – ExampleUnion Operation – Example
Relations r, s:
r s :
A B
1
2
1
A B
2
3
rs
A B
1
2
1
3
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Union OperationUnion Operation
Notation: r s Defined as:
r s = {t | t r or t s}
For r s to be valid.
1. r, s must have the same arity (same number of attributes)
2. The attribute domains must be compatible (e.g., 2nd column of r deals with the same type of values as does the 2nd column of s)
E.g. to find all customers with either an account or a loan
customer-name (depositor) customer-name (borrower)
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Set Difference Operation – ExampleSet Difference Operation – Example
Relations r, s :
r – s :
A B
1
2
1
A B
2
3
rs
A B
1
1
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Set Difference OperationSet Difference Operation
Notation r – s Defined as:
r – s = {t | t r and t s} Set differences must be taken between compatible relations.
r and s must have the same parity
attribute domains of r and s must be compatible
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Cartesian-Product Operation-ExampleCartesian-Product Operation-Example
Relations r, s :
r x s :
A B
1
2
A B
11112222
C D
1010201010102010
E
aabbaabb
C D
10102010
E
aabbr
s
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Cartesian-Product OperationCartesian-Product Operation
Notation r x s Defined as:
r x s = {t q | t r and q s} Assume that attributes of r(R) and s(S) are disjoint.
That is, R S = .
If attributes of r(R) and s(S) are not disjoint, then renaming must be used.
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Composition of OperationsComposition of Operations
Can build expressions using multiple operations
Example: A=C(r x s)
r x s
A=C(r x s)
A B
11112222
C D
1010201010102010
E
aabbaabb
A B C D E
122
102020
aab
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Rename OperationRename Operation
Allows us to name, and therefore to refer to, the results of relational-algebra expressions.
Allows us to refer to a relation by more than one name.
Example:
x (E)
returns the expression E under the name X
If a relational-algebra expression E has arity n, then
x (A1, A2, …, An) (E)
returns the result of expression E under the name X, and with the
attributes renamed to A1, A2, …., An.
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Banking ExampleBanking Example
branch (branch-name, branch-city, assets)
customer (customer-name, customer-street, customer-only)
account (account-number, branch-name, balance)
loan (loan-number, branch-name, amount)
depositor (customer-name, account-number)
borrower (customer-name, loan-number)
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Example QueriesExample Queries
Find all loans of over $1200
Find the loan number for each loan of an amount greater than
$1200
amount > 1200 (loan)
loan-number (amount > 1200 (loan))
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Example QueriesExample Queries
Find the names of all customers who have a loan, an account, or both, from the bank
Find the names of all customers who have a loan and an account at bank.
customer-name (borrower) customer-name (depositor)
customer-name (borrower) customer-name (depositor)
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Example QueriesExample Queries
Find the names of all customers who have a loan at the Perryridge branch.
customer-name (
branch-name=“Perryridge” (
borrower.loan-number = loan.loan-number(borrower x loan)
))
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Example QueriesExample Queries
Find the names of all customers who have a loan at the Perryridge branch but do not have an account at any branch of the bank.
customer-name (
branch-name = “Perryridge” (
borrower.loan-number = loan.loan-number(borrower x loan)
)
) – customer-name(depositor)
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Example QueriesExample Queries
Find the names of all customers who have a loan at the Perryridge branch.
Query 2
customer-name(
loan.loan-number = borrower.loan-number (
(branch-name = “Perryridge”(loan)) x borrower))
Query 1
customer-name (
branch-name = “Perryridge” ( borrower.loan-number = loan.loan-number(borrower x loan)
) )
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Example QueriesExample Queries
Find the largest account balance Rename account relation as d The query is:
(find all balances which are not maksimum, than subtract form account table. A balance value is not maksimum, if it is less than any balance. )
balance (account)
- account.balance (
account.balance < d.balance (
account x d (account)
) )
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Formal DefinitionFormal Definition
A basic expression in the relational algebra consists of either one of the following: A relation in the database A constant relation
Let E1 and E2 be relational-algebra expressions; the following are all relational-algebra expressions:
E1 E2
E1 - E2
E1 x E2
p (E1), P is a predicate on attributes in E1
s(E1), S is a list consisting of some of the attributes in E1
x (E1), x is the new name for the result of E1
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Additional OperationsAdditional Operations
We define additional operations that do not add any power to the
relational algebra, but that simplify common queries.
Set intersection
Natural join
Division
Assignment
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Set-Intersection OperationSet-Intersection Operation
Notation: r s Defined as:
r s ={ t | t r and t s } Assume:
r, s have the same arity
attributes of r and s are compatible
Note: r s = r - (r - s)
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Set-Intersection Operation - ExampleSet-Intersection Operation - Example
Relation r, s :
r s :
A B
121
A B
23
r s
A B
2
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Notation: r s
Natural-Join OperationNatural-Join Operation
Let r and s be relations on schemas R and S respectively. Then, r s is a relation on schema R S obtained as follows:
Consider each pair of tuples tr from r and ts from s.
If tr and ts have the same value on each of the attributes in R S,
add a tuple t to the result, where
• t has the same value as tr on r
• t has the same value as ts on s
Example:
R = (A, B, C, D)
S = (E, B, D)
Result schema = (A, B, C, D, E)
r s is defined as:
r.A, r.B, r.C, r.D, s.E (r.B = s.B r.D = s.D (r x s))
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Natural Join Operation – ExampleNatural Join Operation – Example
Relations r, s :
A B
12412
C D
aabab
B
13123
D
aaabb
E
r
A B
11112
C D
aaaab
E
s
r s :
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Division OperationDivision Operation
Suited to queries that include the phrase “for all”.
Let r and s be relations on schemas R and S respectively where
R = (A1, …, Am, B1, …, Bn)
S = (B1, …, Bn)
The result of r s is a relation on schema
R – S = (A1, …, Am)
r s = { t | t R-S(r) u s ( tu r ) }
r s
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Division Operation – ExampleDivision Operation – Example
Relations r, s :
r s : A
B
1
2
A B
12311134612
r
s
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Another Division ExampleAnother Division Example
A B
aaaaaaaa
C D
aabababb
E
11113111
Relations r, s :
r s :
D
ab
E
11
A B
aa
C
r
s
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Division Operation (Cont.)Division Operation (Cont.)
Property Let q – r s Then q is the largest relation satisfying q x s r
Definition in terms of the basic algebra operationLet r(R) and s(S) be relations, and let S R
r s = R-S (r) –R-S ( (R-S (r) x s) – R-S,S(r))
To see why
R-S,S(r) simply reorders attributes of r
R-S(R-S (r) x s) – R-S,S(r)) gives those tuples t in
R-S (r) such that for some tuple u s, tu r.
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Assignment OperationAssignment Operation
The assignment operation () provides a convenient way to express complex queries. Write query as a sequential program consisting of
• a series of assignments
• followed by an expression whose value is displayed as a result of the query.
Assignment must always be made to a temporary relation variable.
Example: Write r s as
temp1 R-S (r)
temp2 R-S ((temp1 x s) – R-S,S (r))
result = temp1 – temp2
The result to the right of the is assigned to the relation variable
on the left of the .
May use variable in subsequent expressions.
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Example QueriesExample Queries
Find all customers who have an account from at least the “Downtown” and the Uptown” branches.
where CN denotes customer-name and BN denotes
branch-name.
Query 1
CN(BN=“Downtown”(depositor account))
CN(BN=“Uptown”(depositor account))
Query 2
customer-name, branch-name (depositor account)
temp(branch-name) ({(“Downtown”), (“Uptown”)})
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Find all customers who have an account at all branches located
in Brooklyn city.
Example QueriesExample Queries
customer-name, branch-name (depositor account)
branch-name (branch-city = “Brooklyn” (branch))
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Extended RelationalExtended Relational Model Model
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Extended Relational-AlgebraExtended Relational-AlgebraOperationsOperations
1. Generalized Projection
2. Aggregate Functions
3. Outer Join
4. Modification
5. Tuple Relational Calculus
6. Domain Relational Calculus
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1. Generalized Projection1. Generalized Projection
Extends the projection operation by allowing arithmetic functions to be used in the projection list.
F1, F2, …, Fn(E)
E - any relational-algebra expression
F1, F2, …, Fn - arithmetic expressions involving constants and attributes in the schema of E
Example: credit-info(customer-name, limit, credit-balance)
how much money does each person have left to spend?
customer-name, limit – credit-balance (credit-info)
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2. Aggregate Functions and Operations2. Aggregate Functions and Operations
Aggregation function takes a collection of values returns a single value as a result e.g. avg, min, max, sum, count
G1, G2, …, Gn g F1( A1), F2( A2),…, Fn( An) (E)
g - “group” E - any relational-algebra expression
G1, G2 …, Gn - attributes on which to group
• can be empty
Fi - aggregate functions
Ai - attribute names
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Aggregate Operation – Example 1Aggregate Operation – Example 1
Relation r:
A B
C
7
7
3
10
g sum(c) (r)sum-C
27
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Aggregate Operation – Example 2Aggregate Operation – Example 2
Relation account grouped by branch-name:
branch-name g sum(balance) (account)
branch-name account-number balance
PerryridgePerryridgeBrightonBrightonRedwood
A-102A-201A-217A-215A-222
400900750750700
branch-name balance
PerryridgeBrightonRedwood
13001500700
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Result of Aggregate FunctionsResult of Aggregate Functions
Result of aggregation does not have a name
Can use rename operation to give it a name
For convenience, we permit renaming as part of aggregate operation:
branch-name g sum(balance) as sum-balance (account)
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3. Outer Join3. Outer Join
An extension of the join operation that avoids loss of information. Computes the join
adds those tuples from one relation that did not match any tuples in the other relation
Introduces null values: null signifies that the value is unknown or does not exist
All comparisons involving null are (roughly speaking) false by definition.
• Will study precise meaning of comparisons with nulls later
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ExampleExample
Relation loan
Relation borrower
customer-name loan-number
JonesSmithHayes
L-170L-230L-155
300040001700
loan-number amount
L-170L-230L-260
branch-name
DowntownRedwoodPerryridge
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Inner JoinInner Join
Inner Join
loan Borrower
loan-number amount
L-170L-230
30004000
customer-name
JonesSmith
branch-name
DowntownRedwood
300040001700
loan-number amount
L-170L-230L-260
branch-name
DowntownRedwoodPerryridge
customer-name loan-number
JonesSmithHayes
L-170L-230L-155
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Left Outer JoinLeft Outer Join
JonesSmithnull
loan-number amount
L-170L-230L-260
300040001700
customer-namebranch-name
DowntownRedwoodPerryridge
Left Outer Join
loan Borrower
300040001700
loan-number amount
L-170L-230L-260
branch-name
DowntownRedwoodPerryridge
customer-name loan-number
JonesSmithHayes
L-170L-230L-155
Memory aid: “=“ - total (as in E-R diagrams)
The relation on the “=“ side of the operation participates completely in the resulting relation.
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Outer Join – ExampleOuter Join – Example
Right Outer Join loan borrower
loan borrower
Full Outer Join
loan-number amount
L-170L-230L-155
30004000null
customer-name
JonesSmithHayes
branch-name
DowntownRedwoodnull
loan-number amount
L-170L-230L-260L-155
300040001700null
customer-name
JonesSmithnullHayes
branch-name
DowntownRedwoodPerryridgenull
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Null ValuesNull Values
It is possible for the value of an attribute to be null
null signifies: an unknown value
a non-existent valuei.e. no values are meaningful, or appropriate
Computing with null values: The result of any arithmetic expression involving null is null.
Aggregate functions simply ignore null values (convention)
For duplicate elimination and grouping: null is treated like any other value
two nulls are assumed to be the same (convention)S
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Comparing Null ValuesComparing Null Values
Comparisons with null values return the special truth value unknown
“Three-valued logic” using the truth value unknown:
OR: (unknown or true) = true, (unknown or false) = unknown (unknown or unknown) = unknown
AND: (true and unknown) = unknown, (false and unknown) = false, (unknown and unknown) = unknown
NOT: (not unknown) = unknown
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Database Management Systems
4. Modification of the Database4. Modification of the Database
The content of the database may be modified using the following operations: Deletion
Insertion
Updating
All these operations are expressed using the assignment operator.
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DeletionDeletion
A delete request is a query where the selected tuples are removed from the database.
Can only delete whole tuples; cannot delete values on only particular attributes
A deletion is expressed in relational algebra by:
r r – Ewhere r is a relation and E is a relational algebra query. NB E might be a constant relation specifying a single tuple to
be deleted
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Deletion ExamplesDeletion Examples
Delete all account records in the Perryridge branch.
Delete all accounts at branches located in Needham.
r1 branch-city = “Needham” (account branch)
r2 branch-name, account-number, balance (r1)
r3 customer-name, account-number (r2 depositor)
account account – r2
depositor depositor – r3
Delete all loan records with amount in the range of 0 to 50
loan loan – amount 0and amount 50 (loan)
account account – branch-name = “Perryridge” (account)
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InsertionInsertion
To insert data into a relation, we either: specify a tuple to be inserted
write a query whose result is a set of tuples to be inserted
in relational algebra, an insertion is expressed by:
r r E
where r is a relation and E is a relational algebra expression.
The insertion of a single tuple is expressed by letting E be a constant relation containing one tuple.
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Insertion ExamplesInsertion Examples
Insert information in the database specifying that Smith has $1200 in account A-973 at the Perryridge branch.
Provide as a gift for all loan customers in the Perryridge branch, a $200 savings account. Let the loan number serve as the account number for the new savings account.
account account {(“Perryridge”, A-973, 1200)}
depositor depositor {(“Smith”, A-973)}
r1 (branch-name = “Perryridge” (borrower loan))
account account branch-name, account-number,200 (r1)
depositor depositor customer-name, loan-number(r1)
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UpdatingUpdating
A mechanism to change a value in a tuple without charging all values in the tuple
Use the generalized projection operator to do this task
r F1, F2, …, FI, (r)
Each Fi is either
the ith attribute of r, if the ith attribute is not updated, or,
if the attribute is to be updated Fi is an expression, involving only constants and the attributes of r, which gives the new value for the attribute
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Update ExamplesUpdate Examples
Make interest payments by increasing all balances by 5 percent.
Pay all accounts with balances over $10,000 6 percent interest
and pay all others 5 percent
account AN, BN, BAL * 1.06 ( BAL 10000 (account))
AN, BN, BAL * 1.05 (BAL 10000 (account))
account AN, BN, BAL * 1.05 (account)where AN, BN and BAL stand for account-number, branch-name and balance, respectively.
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ViewsViews
In some cases, it is not desirable for all users to see the contents of the entire model
E.g. a person who needs to know a customer’s loan number but has no need to see the loan amount. This person should see a relation described, in the relational algebra, by
customer-name, loan-number (borrower loan)
A relation that is not in the model but is made visible to a user as a “virtual relation” is called a view.
MySQL: Views not available in MySQL 3.23
Prevent access to tables or columns using the access privilege system
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View DefinitionView Definition
A view is defined using the create view statement which has the form
create view v as <query expression>
where <query expression> is any legal relational algebra query expression. The view name is represented by v.
Once a view is defined, the view name can be used to refer to the virtual relation that the view generates.
View definition is not the same as creating a new relation by evaluating the query expression
Rather, a view definition causes the saving of an expression; the expression is substituted into queries using the view.
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View ExamplesView Examples
Consider the view (named all-customer) consisting of branches and their customers.
We can find all customers of the Perryridge branch by writing:
create view all-customer as
branch-name, customer-name (depositor account)
branch-name, customer-name (borrower loan)
customer-name
(branch-name = “Perryridge” (all-customer))
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Updates Through ViewUpdates Through View
Database modifications expressed as views must be translated to modifications of the actual relations in the database.
Consider the person who needs to see all loan data in the loan relation except amount. The view given to the person, branch-loan, is defined as:
create view branch-loan as
branch-name, loan-number (loan)
Since we allow a view name to appear wherever a relation name is allowed, the person may write:
branch-loan branch-loan {(“Perryridge”, L-37)}
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Updates Through Views (Cont.)Updates Through Views (Cont.)
The previous insertion must be represented by an insertion into the actual relation loan from which the view branch-loan is constructed.
An insertion into loan requires a value for amount. The insertion can be dealt with by either. rejecting the insertion and returning an error message to the
user. inserting a tuple (“L-37”, “Perryridge”, null) into the loan relation
Some updates through views are impossible to translate into database relation updates
create view v as branch-name = “Perryridge” (loan))
v v (L-99, Downtown, 23)
Other updates cannot be translated uniquely all-customer all-customer {(“Perryridge”, “Johnson”)}
• Have to choose loan or account, and create a new loan/account number!
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Tuples Inserted Into Tuples Inserted Into loan loan and and borrowerborrower
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Views Defined Using Other ViewsViews Defined Using Other Views
One view may be used in the expression defining another view
A view relation v1 is said to depend directly on a view relation v2 if v2 is used in the expression defining v1
A view relation v1 is said to depend on view relation v2 if either v1
depends directly to v2 or there is a path of dependencies from v1 to v2
A view relation v is said to be recursive if it depends on itself.
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View ExpansionView Expansion
A way to define the meaning of views defined in terms of other views.
Let view v1 be defined by an expression e1 that may itself contain uses of view relations.
View expansion of an expression repeats the following replacement step:
repeatFind any view relation vi in e1
Replace the view relation vi by the expression defining vi
until no more view relations are present in e1
As long as the view definitions are not recursive, this loop will terminate